A circuit for compensating base current of a bipolar transistor is provided in an input section of an amplifier circuit, for example. The circuit is effective in reducing base current of a bipolar transistor to be compensated which is connected with an input terminal of the amplifier circuit. The circuit is also effective in raising input impedance of the amplifier circuit.
A conventional circuit for compensating base current is provided with a bipolar transistor to generate a compensating current. The bipolar transistor has the same amplification characteristic as the bipolar transistor to be compensated which is connected with an input terminal of an amplifier circuit.
A current Ie2 is flowed through an emitter of the bipolar transistor for generating a compensating current. The current Ie2 has the same amount as an emitter current Ie1 of the transistor to be compensated. Further, a current Ib2 which flows through a base of the transistor for generating a compensating current is mirrored by a current mirror circuit. The obtained mirror current flows into the base of the transistor to be compensated.
In this case, assuming that a current which flows into the base of the transistor to be compensated from the input terminal is denoted by “Iin”, a base current Ib1 of the transistor to be compensated is expressed by “Ib1=Iin+Ib2”. Accordingly, the current Iin which flows from the input terminal is expressed by “Iin=Ib1−Ib2”.
The emitter current Ie2 of the transistor for generating a compensating current has the same amount as the emitter current Ie1 of the transistor to be compensated. Since a collector-emitter voltage Vce2 of the former transistor is same as a collector-emitter voltage Vce1 of the latter transistor, the base current Ib2 of the former transistor becomes same as the base current Ib1 of the latter transistor, i.e., Ib2=Ib1. From the above formula, the current Iin is obtained as “Iin=Ib1−Ib2=0”, namely the current Iin which flows from the input terminal is zero, and the input impedance becomes high.
The emitter potential of the transistor to be compensated is a potential which is dropped from an input voltage applied to the input terminal, by an amount of a base-emitter voltage of the transistor. The collector-emitter voltage Vce1 fluctuates in accordance with fluctuation of the input voltage.
Accordingly, the base current Ib1 of the transistor to be compensated fluctuates in accordance with fluctuation of the applied voltage due to an early effect. On the other hand, since the emitter potential of the transistor for generating a compensating current is not influenced by the applied voltage, the collector-emitter voltage Vce2 of the transistor for generating a compensating current does not fluctuate even when the applied voltage fluctuates. As a result, the base current Ib2 of the transistor for generating a compensating current does not fluctuate even when the applied voltage fluctuates.
The conventional circuit for compensating base current has a problem that the subtraction of the base current Ib1 from the base current Ib2 becomes not equal to zero (“Ib2−Ib1≠0”) when the voltage applied to the input terminal fluctuates. As a result, the current Iin which flows from the input terminal fluctuates depending on the applied voltage.